Torque Vs Speed - How Much Do I Need?

How-Tos & Esk8 mechanics
#63

My build is going to be using flipsky 6354 190kv motors, with a 16T motor pulley and 60T on 150mm pmeummies. Is this gearing too torque biased?

#64

The only thing that has anything to do with torque is amps and gear reduction, voltage has 0 bearing on either

If you keep your motor power the same, say 5kw but you raise your voltageā€¦your torque actually goes down due to P=EI

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#65

how much motor current? how much do you weigh~?

#66

Im planning to use Molicel p42As in 12s4p configuration and allow the motors to draw 50-60A. I weight ~70kg

#67

i definitely wouldnt say that setup will be too torque biased. its not even an especially torquey setup with that current tbh.

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#68

If you increase the voltage and leave the motor & battery current limits the same, torque will be the same at low speeds and higher than before at mid to high speeds. Thatā€™s because the same battery current with higher voltage corresponds to more watts at the same speed, which equates to higher motor current at the same speed.

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#69

as i understand it for a given set of hardware, the voltage is constant for a given speed, regardless of what the max voltage the battery can supply is. so when you change voltage, you wont get different torque unless you change gearing, in which case torque will be uniformly increased across all speeds

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#70

On 12s, thatā€™s a good starting point. You can always swap in a 18t or 20t pulley if you find yourself wanting more speed when the battery sags.

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#71

Do you reckon the motors will be a bottleneck?

#72

Absolutely not. Run them at 50-60amps and dial in your preferences with the throttle curve.

Edit: how much do you weigh? If you are 200+ pounds, maybe go for bigger motors that can take more continuous amps.

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#73

:poop:

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#74

Am I missing something? That seems like a perfectly reasonable response @J0ker

#75

I just needed something to post so it was poop.

#76

Lol, I missed that.

Yeah, @That_Jamie_S_Guy 6355ā€™s are a good choice. You could go for bigger motors, but it all depends what you want the board to be like.

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#77

If your battery current limit is set lower than the motor current limit, you will only be able to hit the motor current limit at low speeds. (Close to maximum speed the max motor current will be much closer to the battery current limit, often well below the motor current limit.) Now if you increase the battery voltage, keeping the same battery current limit, at the same speed the same battery current equates to more motor current than before (because itā€™s more watts). So when you increase the battery voltage you will have more acceleration available at the higher rpm range, if you leave the current limits unchanged and the battery current limit is less than the motor current limit. Or you can get the same acceleration profile as before if you decrease the battery current limit when you increase the battery voltage, except youā€™ll be able to reach a higher top speed. The unloaded rotor stops accelerating when the back emf voltage from the moving magnets equals the battery voltage, and this rotor speed is higher when the battery voltage is higher, regardless of the current limits.

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#78

the thing is that voltage isnā€™t actually changing. rpm of a bldc motor is directly proportional to voltage, so given a wheel size, gearing, and motor kv, voltage for a given speed will be exactly the same. As I understand it, BLDC motor controllers are essentially able to regulate RMS voltage, and this is how they control speeds. Justbecause the controller could get to a higher max voltage (at the motor) than before, that doesnā€™t mean that voltage at every speed increases. Instead, only maximum speed is increased. (and at these newly achievable speeds, voltage will be higher than was possible before).

I hope Iā€™m not spreading misinformation, can someone who actually has EE training weight in on this? maybe @jaykup or @linsus ?

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#79

No.

w/ BLDC itā€™s-

(PWM_Effective_Voltage - Back_Emf_Voltage) / Winding_2_Phase_Resistance_Ohms = Motor_Current

RPM / KV = Back_Emf_Voltage

Battery_Voltage * Duty_Cycle = PWM_Effective_Voltage

To approximate the battery wattage you can take:

((Motor_Current)^2 * Winding_2_Phase_Resistance_Ohms) + (Motor_Angular_Speed_RADSEC * Torque_NM)

where:

Torque_NM = KT * Motor_Current

where:

KT = 1/KV_in_RADSEC_per_volt

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